Video reports filed by Karina Kling, Heidi Zhou and Alana Rocha.

What started out as a normal day for some students and faculty quickly turned hectic.

Police say 19-year-old Colton J. Tooley opened fire on the University of Texas campus around 8 a.m. Tuesday. Authorities said Tooley fired shots with an AK-47 on the sixth floor of the Perry-Castaneda Library. He then shot and killed himself. No one else was shot or injured during the incident.

Student Anthony Smith was at the library studying with several others with the shootings happened.

"We went to the patio café, like we normally do. [It’s] just outside the library, and as we were leaving, going to class, that's when things went from normal to insane," he said. "I heard a loud bang. One of my friends ran out, saying he heard four gunshots. That's when we saw people running from the library."

Students remained on edge throughout the day.

"Just seeing people from the library like that – it's a quiet place," Smith said. "People are trying to get in. Today is test day, so people are on edge; people have been up all night."

Law enforcement officials conducted a sweep through the rest of the campus after early suspicions of a second suspect.

Junior Anna Pope had just got off the bus when she was pushed in Benedict Hall and put on lockdown.

"Mostly I was curious. I just wanted to know what was going on. No one was telling us anything. They were just locking us up in the building. That just causes more problems, because some people are just so worried and nervous. I just wanted to know what was happening," Pope said.

Just moments after the shots were fired, the university sent out an alert to 43,000 students, faculty and staff telling them not to come to campus or to stay where they were, if they were already there.

University junior John Dixon said he received about nine alerts from the system while he was in class at Calhoun Hall.

"We kind of brushed it off and continued with our lecture," Dixon said. "The T.A., he was kind of concerned, but I think he was more concerned with getting the material done, because there were a lot of people laughing and making jokes, and he got to the point where he was like, 'Let’s do what we came here to do.'"

The alert system also sounded emergency sirens and loudspeakers throughout the campus.

"We heard searches going on, and we heard people banging on doors, and the SWAT team giving each other orders. I’ve never been in a situation like that. It was really nerve racking," student Kenny Thornally said.

Students used their phones to call and text their emotions and experience.

"Everyone was franticly calling their loved ones and friends, trying to get more information and let everyone know they were safe for the moment," student Jonathon Rodriguez said. "Rumors were flying around like crazy. Texts were coming in from other people on campus." (Article continues after slideshow)

Authorities, including the FBI, SWAT, Texas Rangers, along with many local agencies, descended on the campus in attempts to stop a reported second suspect.

"A lot of people got messages from their families, saying that the news said that someone was in Calhoun. Then someone said that Texas sent out a warning, saying to stay away from the area and that there might be a possible shooter there," student Eric Davidson said.

"SWAT came in and it just seemed surreal, because they had all of their guns, and they were ready to go. They were just decked out and everything. It really freaked everyone out and that’s when we all sort of banded together to help barricade it and everything."

Students banded together and used their own possessions to barricade themselves inside Calhoun Hall.

"They had us barricade our doors, and we had nothing to barricade them with because there are six doors in there. So me and my friend had to use our belts to barricade the doors. We had to use trash cans [and] whatever we could," Davidson said.

One professor said students' safety comes first, but called the lockdown "an inconvenience."

"It's a pain for me. It's negative for the education of our students. We usually don't think about the spillover effects of something like this, but something like this - because of the shutdown - effects everybody, disturbs everybody's lives in the whole area," faculty member Daniel Hamermesh said.

Students said the incident made them think about their environment.

"I got up thinking it was a normal, somewhat stressful day with three tests. Now the tests are the last thing I can think about," student Jessica Field said. "It's scary. I couldn't help but think of all of the recent shootings, or back to the 60s when the tower shooting happened. It’s really scary. It makes you not feel safe in a place that you come every day."

Pope agreed.

"I heard about Charles Whitman, back in the day, and it’s kind of weird that that would happen again. I never thought it would happen when I was here,” she said. “I’m just glad nobody that got hurt. It’s just another day in the world. It’s kind of a crazy place these days."

Faculty and staff said they are trained to be prepared during these scenarios.

"As a UT employee, we are constantly aware of the possibility of a disturbed individual over whelmed with the pressures of school and/ or work," employee Jess Moss said.

But Moss said nothing can prepare you for the situation.

"It's just another reminder how little we know about each other, and that we have to return to campus aware of the dangers, but also confident in each other."

The campus will be back open Wednesday. All classes are expected to resume as normal.

Online Producer Kevyn Oakes contributed to this report.